This invention relates to high temperature furnaces, i.e. furnaces operating at temperatures in excess of about 1300xc2x0 C. and preferably in excess of about 1700xc2x0 C. (about 3,100xc2x0 F.) in air and inert atmospheres. At 1700xc2x0 C. numerous semi-high temperature metals, i.e. those that melt without decomposition or reaction at between 1000xc2x0 C. and 1700xc2x0 C. in an inert atmosphere, can be processed. Examples of such semi-high temperature metals include beryllium, copper, cobalt, gadolinium, gold, neodymium, palladium, uranium, iron, manganese, nickel, silicon, titanium, and yttrium. Further, at temperatures up to 1700xc2x0 C. certain relatively low temperature glass and ceramic materials such as manganese oxide, aluminum metasilicate, silicon dioxide (quartz) and sodium aluminum silicate can be processed.
The invention more particularly relates to such ultra-high temperature furnaces that are low cost, reliable and that require relatively low maintenance.
Major problems exist with current high temperature furnaces that are usually electrical type furnaces using electric arcs, which are unstable, difficult to control, and unpredictable or electric resistance furnaces which are deficient in that at high enough temperatures rapid disintegration of the electrode or disintegration of electrical contacts occurs due to oxidation or thermal shock associated with non-uniform heating and cooling. Even assuming such problems did not exist, such furnaces are not available that can reliably and controllably operate at ultra high temperatures, e.g. in excess of 3000xc2x0 C.
In accordance with the invention, an induction furnace is provided that has a plurality of high temperature electrically conductive ceramic electrodes having no connecting electrical lead (leadless electrodes). The leadless electrodes are exterior to and proximate a working furnace space. At least one metallic electrical conductor surrounds but is not connected to the ceramic electrodes and a power supply is connected to the at least one electrical conductor so that activation of the power supply creates an alternating current through the electrical conductor having a frequency of between 102 and 1011 cycles per second and of sufficient energy to create an electromagnetic flux having a wave length of between about 10xe2x88x921 and about 108 cm of sufficient flux density to heat the ceramic electrodes to a temperature in excess of about 1300xc2x0 C. and preferably above about 1700xc2x0 C. to heat the space. In general a plurality of electrodes collectively surround the furnace space, e.g. in the form of a plurality of spaced parallel rods. When the material being heated within the furnace is air sensitive or the temperature is so high that the electrodes will be affected by surrounding air, the electrodes and contents are surrounded by an inert atmosphere selected from a vacuum, nitrogen, the noble gases or mixtures thereof. In such a case, temperatures in excess of 2000xc2x0 C. may be reached.
A very high temperature furnace for operation in air is included within the invention wherein the electrodes proximate the working furnace space are made of a high temperature, stable, electrically conductive metal oxide. A preferred metal oxide for that purpose is a metal oxide that is conductive at high temperatures, e.g. zirconia. In such a case, at least one intermediate leadless electrode is provided sufficiently near the proximate metal oxide (e.g. zirconia) ceramic electrode to heat the proximate metal oxide ceramic electrodes above their electrical conducting temperature, i.e. the temperature at which the oxide becomes sufficiently electrically conductive so that it conducts sufficient current to maintain its own temperature, usually above about 1000xc2x0 C. The intermediate electrode is conductive at low temperatures, e.g. at room temperature, and may be ceramic, e.g. silicon carbide or a high temperature metal, e.g. platinum. The intermediate electrode is heated by the flux to a temperature above the electrical conducting temperature of the proximate oxide electrode and is withdrawn from the flux when the proximate oxide electrodes become electrically conductive so as to maintain its own temperature above its conductive temperature within the flux. Since the proximate electrodes, e.g. zirconia, are already an oxide, this permits the zirconia electrodes to act as the heating electrodes to an ultra high temperature in air. In such a furnace a zirconia electrode may be heated to above 2200xc2x0 C. by the flux thus similarly heating the furnace space. In a preferred embodiment, the proximate electrodes are essentially made of zirconia and a plurality of intermediate leadless ceramic electrodes are either provided between the proximate ceramic electrode and the electrical conductor or between the proximate ceramic electrodes. The intermediate ceramic electrodes can then be heated by the flux to a temperature above the electrical conducting temperature of the zirconia, i.e. to about 1200xc2x0 C., and can then be withdrawn.
The invention further includes the method of heating a material to high temperature using the furnace of the invention.